This application claims the priority of German Patent Document 100 38 743.8, filed Aug. 9, 2000, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an arrangement for igniting a combustible gas mixture for the exhaust system of an internal-combustion engine and to a corresponding exhaust system.
When aftertreating the exhaust gas of an internal-combustion engine of a motor vehicle, the reduction of pollutants in the exhaust gas during the cold start and the warm-up of the internal-combustion engine has priority as the most promising potential for reducing pollutants. Among others, methods are known for this purpose in which, during the cold start, a combustible air/fuel mixture is generated in the exhaust gas train and the heating value contained in the exhaust gas is utilized for the rapid heating of the pollutant-converting catalysts to their starting temperature or usage temperature. This takes place in that the combustible air/fuel mixture is ignited again at an appropriate point in the direct proximity of the catalyst.
The ignition devices known for the ignition of such a combustible gas mixture, for example, spark plugs or glow plugs, are unsuitable for this task or can only be used to a limited extent because, under the existing environmental conditions, such as low pressure, low temperature and a high fraction of natural gases, a reliable ignition and complete combustion cannot be ensured.
It is therefore an object of the present invention to provide an arrangement for igniting a combustible gas mixture for the exhaust system of an internal-combustion engine which can reliably carry out the ignition and offers a high probability of a complete combustion.
The arrangement and corresponding exhaust system according to preferred embodiments of the invention have the advantage that a reliable ignition is ensured and the warm-up time of the catalyst and the exhaust gas emissions can be drastically reduced. The required power is low because only the heating wire and not the entire carrier material must be heated. According to preferred embodiments of the invention, there is provided an arrangement for igniting a combustible gas mixture for the exhaust system of an internal-combustion engine, comprising a frame device which is wound with a heating wire and can be installed in an exhaust gas train of an exhaust system transversely to a flow axis of exhaust gas in the exhaust system, and a power supply device for supplying the heating wire with electric current for generating a predetermined heating wire temperature, wherein the frame device defines a flow range in which a heating wire grid is provided.
According to certain preferred embodiments of the invention, there is provided an exhaust system comprising an ignition arrangement of the type described herein installed transversely to a flow axis of exhaust gas flowing in the system, a first catalyst device arranged downstream of the ignition arrangement, and a housing which has a widening for receiving a frame device of the ignition arrangement.
An idea on which the present invention is based includes arranging a grid-shaped structure of heating wire or resistance wire as the ignition arrangement transversely to the main flow axis in the exhaust gas train and of electrically heating it to a temperature which is suitable for igniting the oncoming combustible gas mixture. Expediently, the material thickness of the conductive heating wire should be selected to be as small as is possible with respect to the strength in order to minimize the heating thermal capacity and thus the required electric current intensity.
Advantageous further developments and improvements of the invention are described herein and in the claims.
According to a preferred further development of certain preferred embodiments of the invention, the frame device comprises a first frame and a second frame which is arranged essentially parallel thereto in a spaced manner, which frames are alternately wound with the heating wire. This results in a stable heating wire grid.
According to another preferred further development of certain preferred embodiments of the invention, the first frame can be acted upon by the power supply device by a first electric potential (plus pole), and the second frame can be acted upon by a second electric potential (ground potential). This embodiment with two frames electrically provides a parallel connection of many individual heating wire elements and has the advantage of a minimized electric resistance. Furthermore, the tearing-off of a heating wire segment at any point does not cause a failure of the entire ignition arrangement.
According to another preferred further development of certain preferred embodiments of the invention, the first and the second frame are separated from one another by a preferably elastic insulation device. This provides the required electric uncoupling and a mechanical prestressing against the thermal expansion.
According to another preferred further development of certain preferred embodiments of the invention, the heating wire is threaded by way of hook ends provided on the frame device.
According to another preferred further development of certain preferred embodiments of the invention, the heating wire is prestressed by way of an elastic prestressing device for compensating the thermal expansion.
According to another preferred further development of certain preferred embodiments of the invention, the heating wire is catalytically coated. This reduces the heating temperature and promotes the catalyst effect.
Exhaust systems according to preferred embodiments of the invention have an ignition arrangement installed transversely to the flow axis of the exhaust gas has a first catalyst device arranged downstream of the ignition arrangement and a housing which preferably has a widening for accommodating the frame device.
According to another preferred further development of certain preferred embodiments of the invention, the ignition arrangement is arranged such that the flow area corresponds essentially to the cross-section of the catalyst device. This results in a whole-surface heating of the catalyst device.
According to another preferred further development of certain preferred embodiments of the invention, the ignition arrangement is arranged such that a flame front of the ignited gas mixture can form before the catalyst device is reached. This results in a maximal efficiency of the preheating.
According to another preferred further development of certain preferred embodiments of the invention, the frame device is connected with the housing on its circumference. This prevents a flow around the ignition arrangement.
According to another preferred further development of certain preferred embodiments of the invention, a second catalyst device is arranged upstream of the ignition arrangement. It is particularly effective to place the ignition arrangement in the housing of the main catalyst in the free space between a first and a second monolith.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.